1. Technical Field
The present invention relates to a droplet ejection apparatus.
2. Background Art
An ink jet printer, which is one type of droplet ejection apparatus, forms an image on a predetermined sheet of paper by ejecting ink drops (droplets) via a plurality of nozzles of a printing head of the ink jet printer. The printing head (ink jet head) of the ink jet printer is provided with a number of nozzles. However, there is a case where some of the nozzles are blocked due to an increase of ink viscosity, intrusion of air bubbles, adhesion of dust or paper dust, or the like, and therefore these nozzles become unable to eject ink droplets. When the nozzles are blocked, missing dots occur within a printed image, which results in deterioration of image quality.
As far, a method of optically detecting a state where no ink droplets are ejected through the nozzles of the ink jet head (a state of failing ink droplet ejection) for each nozzle of the ink jet head was devised as a method of detecting such an ejection failure of an ink droplet (hereinafter, also referred to as the missing dot) (for example, Japanese Laid-Open Patent Application No. Hei. 8-309963 or the like). This method makes it possible to identify a nozzle causing the missing dot (ejection failure).
In the optical missing dot (droplet ejection failure) detecting method described above, however, a detector including a light source and an optical sensor is attached to a droplet ejection apparatus (for example, an ink jet printer). Hence, this detecting method generally has a problem that the light source and the optical sensor have to be set (or provided) with exact accuracy (high degree of accuracy) so that droplets ejected through the nozzles of the droplet ejection head (ink jet head) pass through a space between the light source and the optical sensor and therefore intercept light from the light source to the optical sensor. In addition, since such a detector is generally expensive, the droplet ejection apparatus having the detector has another problem that the manufacturing costs of the ink jet printer are increased. Further, since an output portion of the light source or a detection portion of the optical sensor may be smeared by ink mist through the nozzles or paper dust from printing sheets or the like, there is a possibility that the reliability of the detector becomes a matter of concern.
Further, the droplet ejection apparatus that carries out the optical missing dot detecting method described above detects a missing dot, that is, ejection failure (non-ejection) of ink droplets through the nozzles when the droplet ejection apparatus does not record (print) an image on a sheet of paper. Since the droplet ejection apparatus cannot detect such a missing dot when recording (printing) an image on a droplet receptor (droplet receiving object) such as a sheet of printing paper, there is a problem that the droplet ejection apparatus cannot determine (detect) whether or not a missing dot (absence of a pixel) actually occurs on the printed image or the like.